Radially expandable mandrels find utility in a number of environments including the manufacture of tires, bands and laminated products and the restoration of collapsed tubular structures to their original shape.
In tire manufacture, uncured rubber compound and ply materials are wrapped in layers around a tire building drum when the drum is in an expanded cylindrical condition in order to form what is known in the art as a tire "carcass." While on the drum, the carcass is typically subjected to an operation known as "stitching" in which the drum is rotated and the carcass is forcibly pressed between the drum and external rollers in order to eliminate air pockets or gaps in the layers and to generally coalesce the carcass into a substantially integral structure. When the carcass is completed, the drum is driven into an at least partially radially collapsed condition in order to facilitate removal of the carcass for further processing.
A typical tire building drum includes a plurality of alternate wide and narrow drum segments fitted with arcuate shells on their outer surfaces. The drum segments are coupled to an internal mechanism capable of driving the drum segments radially in opposing directions. When a carcass has been completed, the segments are retracted radially inwardly in order to at least partially collapse the drum to permit the carcass to be readily removed. As in the case of U.S. Pat. No. 3,779,835, the mechanism may be one which times the retraction of the narrow segments to proceed ahead of retraction of the wide segments in order that the wide and narrow segments do not interfere with one another and possibly jam the mechanism. After the previously-built carcass has been removed, the segments are then extended radially until the drum assumes an expanded condition in which the shells mate to form a substantially perfect cylinder upon which the next carcass can be built.
Tire manufacturers constantly strive to improve the uniformity of the tires they make so that those tires will be less likely to generate objectionable vibration or steering disturbances when they are put to use on vehicles. In order to be capable of consistently and economically producing tires capable of meeting increasingly stringent uniformity specifications, it is believed important that tire building drums be capable of simultaneously meeting a number of performance criteria. However, prior art tire building drums tend to suffer from deficiencies in one or more of the areas which will now be discussed.
In the first instance, tire building drums represent a considerable capital investment to tire manufacturers. As such, it is important that their structure be relatively simple so that they can be economically produced and maintained. In view of the investment they represent, the construction of tire building drums should also be one which lends itself to a long operating life with low wear. The problem of wear is not merely an issue of capital expense however.
A problem from which a number of prior art tire building drums suffer is that they are sometimes subject to sliding wear of components which influence either the expanded diameter of the drum or the precise fit of adjacent shells when the drum is in its expanded condition. Variability of the expanded diameter of the drum over it useful life is liable to cause undesired variation in tires produced using the drum and gaps between segments can generate scrap as the result of rubber compound being forced into such gaps.
Lack of rigidity and radial stability is also a problem with some prior art tire building drums, particularly after they have been in use for some time. It is important that a tire building drum be substantially rigid when the drum is expanded so as to be capable of resisting external forces during stitching or other operations without excessively deforming in a way which could effect the quality of tire carcasses built on the drum. For safety as well as quality reasons, a tire building drum should exhibit a high degree of radial stability, particularly when expanded, so as not to be susceptible to unexpected collapse.
Yet another performance deficiency of at least some prior art tire building drums relates to their tendency to exhibit backlash. Backlash can be described as a lack of predictability of the radial positions of the exterior drum segments for any given condition of the mechanical inputs which drive the drum. Such mechanical inputs are supplied by a tire building machine to which a tire building drum is connected in use and conventionally take the form of relative rotary inputs. For precise control over an important phase of the tire manufacturing process, it is desirable for a tire building drum to accurately translate its rotary inputs into predictable and repeatable radial positioning of the exterior drum segments. When backlash is present however, the radial position of the segments for a given rotary input is somewhat uncertain. There is concern that such uncertainty could be capable of causing undesired variability in tire carcasses built on the drum. It is desirable therefore that a tire building drum be substantially devoid of backlash and remain so irrespective of normal wear accruing over its operating life.
In addition to the foregoing, a tire building drum should have an operating mechanism which is sufficiently compact as to be capable of operating a drum small enough to produce carcasses for thirteen inch or even smaller tires without sacrificing performance in the areas noted above. The relative complexity and size of components needed for adequate strength in some prior art tire building drums prevents their use or degrades their performance in such applications.